The present invention relates to a system for drawing water from contaminated wells and treating the water drawn thereof.
The availability of safe drinking water is rapidly decreasing due to contamination of sweet water reserves, brought about by excessive non-sustainable supply. As a result, the water from these sources often needs to be purified before it can be used for potable purposes. This process of purification is referred to as water treatment.
A typical water treatment process is a two-stage process. The first stage is the drawing of water from the natural/artificial sources such as wells, rivers and even the sea. Water is drawn from these sources using various types of devices. Specifically, for well water, well pumps are used to draw water from the well to the ground level. Water drawn from the well is often saline in nature. Hence, the second stage of water treatment is the process to remove contaminants and dissolved salts from the water obtained in the first stage.
The second stage is further segmented into various processes. In particular, water is first filtered to remove large sized contaminants such as silt, and various microorganisms. This stage of water treatment mimics the natural filtration of water as it moves through the ground. Filtration is followed by various treatments such as the application of chemical disinfectants and/or UV radiation so as to kill or neutralise the more dangerous bacterial and viral contaminants.
The remaining solutes and active contaminants that have not yet been removed or neutralised are extracted, where possible, using various forms of membrane technology, dialysis etc. The salt component removal is referred to as the process of desalination. Desalination is one of the more costly and high energy aspects of the water treatment process. Hence, improvements in the desalination process have a substantial impact on the availability of water. Desalination can be performed using a number of techniques, inter alia reverse osmosis (RO).
Reverse osmosis based desalination consumes relatively little energy and is gaining popularity for small and medium scale desalination. A RO-based desalination unit comprises a high-pressure pump, a module divided into two chambers by a semi-permeable membrane and a pressure control unit. The saline water is pumped into the module using a pressure-amplifying device such as a high-pressure pump. The semi-permeable membrane permits a water flux across the membrane, but inhibits the transport of salts. The water (permeate) in the low-pressure chamber beyond the membrane is desalinated, and the salt is left behind in the high-pressure chamber in front of the membrane. The concentrated salt water in this high-pressure chamber leaves the module via a pressure control valve. The desalinated water (hereinafter referred to as sweet water) can easily be polished for various end uses such as drinking or agricultural purposes. The concentrated salt solution (hereinafter referred to as brine) is the waste product that requires disposal.
The brine produced in the RO-based desalination process has a higher salt concentration than that of the feedstock. Unless there is a clear use for this brine it can represent a serious nuisance value, as it has to be disposed of. In particular, the brine cannot be poured onto the land or allowed to contaminate sweet water reserves. Also, the energy stored in the high-pressure brine line is lost if the brine is just jetted into the environment. One way of avoiding the energy loss is to use a hydraulic energy recovery system mounted on the brine line. This solves the problem of substantial energy recovery, but does not provide a solution concerning the brine disposal.
Such energy recovery systems are described e.g. in U.S. Pat. No. 6,540,487 and GB-02 363 741.
Whilst the energy stored in the brine solution is used to reduce energy consumption in the desalination unit, the brine solution still needs to be disposed of after passing through the energy recovery unit. Hence, there is a need for a method and system, which avoids the disposal of the brine solution into the environment.
Apart from the problem of disposal of the brine solution, there are various other problems with the existing water treatment processes. In particular, there are problems associated with the method used for drawing water from a deep well.
Deep wells, such as those that would have to be tapped in many inland arid areas, require the lowering of an electrically driven pump down to the bottom of the well. The performance of the pumps is dependent on the bore size of the well. In particular, a large borehole permits the use of a sufficiently large diameter pump to cope with a high flow rate. However, for deep wells it is unrealistic to drill bore sizes that exceed 200-300 mm and so the mechanical pump that can be lowered down is limited in performance. Thus, the volume of water that can be drawn from a deep well is not very high even if the aquifer has a very large capacity. This problem is very prominent in places far away from the sea and especially in arid regions, which have scarce water resources. The water table in such areas is often found to be very low down necessitating deep wells. Limited water resources imply that these wells must be fully utilised and run at flow rates that are sustainable and yet large enough to satisfy the water requirement.
Furthermore, the known pumps generally comprise moving parts and in consequence run a risk of contamination either by lost lubricants or by wear of the moving parts.
Like all machines having moving parts, they need some kind of maintenance and have a limited lifetime.